Reaction Pathways and Intermediates of Perfluoroethyl Groups Adsorbed on Cu(111)

by Huang, Jia-Tze

Abstract (Summary)

We investigated the reactivity and bonding of perfluoroethyl groups (C2F5) on Cu(111) under ultra high vacuum conditions. Perfluoroethyl moieties bonded to the surface were generated by the dissociative adsorption of perfluoroethyl iodide. Temperature-programmed reaction/desorption (TPR/D) and reflection- adsorption infrared spectroscopy (RAIRS) revealed abounding reaction pathways, and a variety of intermediates were either identified or inferred. The major desorption products, hexafluoro-2-butyne and hexafluorocyclobutene, were detected at 360K and 440K, and some octafluorobutene was observed at 320K at higher coverages, implicating that two fluorine atoms were abstracted step-by-step from the C2F5 on Cu(111). Two sets of signature IR bands were recognized. One set (2054cm-1, 1409cm-1, 1210cm-1) was found to correlate with the surface-bound trifluorovinyl moieties which were also confirmed by directly generating this species from trifluorovinyl iodide. The other set of vibrational features (1322cm-1, 1224cm-1, 950cm-1) presumably implied the trifluoro- ethylidyne intermediate on the surface. Hence, C2F5(ad) underwent the £\-F and £]-F elimination reactions in sequence to yield trifluorovinyl which eventually led to hexafluoro-2-butyne. The alternative route was that C2F5(ad) proceeded via the £\-F elimination reaction twice to render trifluoroethylidyne which ultimately resulted in hexafluorocyclobutene. To our knowledge, the occurrence of the sequential £\-F and£]-F elimination pathway, or the double £\-F elimination reaction has never been observed in any single system.